1. Field of the Invention
This invention relates to microstrip/stripline transmission lines and microstrip/stripline filters and to a method of construction thereof. More particularly, this invention relates to filters and transmission lines having at least a portion thereof divided into elongated strips.
2. Description of the Prior Art
Microstrip or stripline filters are an important part of microwave circuit designs. Generally, these filters are used in low Q and low power applications because firstly, the conventional conducting materials, for example, gold, silver, copper, etc. are relatively lossy and, secondly, the cross-section current distribution of a microstrip/stripline filter is highly non-uniform. High Q can be achieved for narrow band microstrip/stripline filters when they are constructed of high temperature superconductive (HTS) materials. HTS materials improve the power handling capability of these filters as they have a low loss and high current capacity. It is known to provide filters with improved power handling capability by using low impedance lines and dual-mode patch resonators. Microwave filters using dual-mode patch resonator structures can handle more power than single mode line resonator filters because of the patch size. However, there are limitations on the layout and therefore the size of the filter.
In a paper by Liang, et al., entitled "High-Power HTS Microstrip Filters for Wireless Communication" and published in IEEE MTT-S International Microwave Symposium, High Power Superconducting Microwave Technology Workshop Notes, May, 1994, several narrow band filters are described for high power handling. These filters use low impedance line (i.e. wider resonator line width) to reduce the current density inside the resonator. For a five-pole 0.6% filter with two GHz center frequency, 30 dBm input power at 77K and 41 dBm input power at 12K have been attained. However, increasing the line width of a resonator can reduce the average cross-section current density, but it cannot effectively reduce maximum current density since the cross-section current density distribution of a microstrip or stripline is highly non-uniform.
It is known that the current concentrates more towards the outer surface of a round transmission line when frequency becomes higher. The effective current carrying area of the line cross-section is limited to the outer surface. It is known that microstrip/stripline transmission lines or filters have a non-uniform current distribution and that significantly higher current density exists near the edge of the line in what can be referred to as the "edge effect".